A charged particle instrument uses electrons that interact with a specimen to gain information from the specimen. Examples of such instruments are transmission electron microscope, atomic force microscopes, atom probe field ion microscopes and devices incorporating other scanned probe and x-ray technology for high magnification and imaging. Additionally, high angle annular dark field detections may be utilized in conjunction with such devices for high resolution scanning or transmission electron microscopy. In order for a specimen to be viewed using these devices, and more particularly, a transmission electron microscope, or TEM, it must have a portion or area that is electron transparent and atomically clean, meaning it is on the order of one atomic layer to 5 microns thick, depending on the material and the accelerating voltage of the TEM. One method of creating an electron transparent area in a specimen involves first mechanically reducing the size of the specimen in a gross fashion utilizing cutting, cleaving, thinning or polishing techniques, such as with a dimpling grinder or wedge polisher, and then ion milling the specimen. In ion milling, one, or preferably two, ion beams comprised of an inert gas, such as argon, are generated by an ion beam source or sources, otherwise known as ion guns, and are aimed at the mechanically reduced portion of the specimen. In some instances, corrosive beams may also be utilized for specific reduction or modification of the specimen material. Preferably, one ion beam is aimed at the top of the specimen at an angle of approximately 5-10° from horizontal, and a second ion beam is aimed at the bottom of the specimen at an angle of approximately 5-10° from horizontal. The ion beams remove material from the specimen by momentum transfer. Typically, ion milling is used to create a small hole in the center of the already mechanically thinned portion of the specimen such that the portions of the specimen adjacent to the hole are electron transparent. The ion beams used in conventional ion milling are on the order of 250 μm-2 mm in diameter, and have ion energies on the order of 0.5-10 keV, accomplishing material removal, or milling rates on the order of 20 μm/hr. Conventional ion milling has been accomplished utilizing lower energy devices, typically in the 50-100 eV range, but devices designed for this low energy utilization are frequently incapable of developing higher energies with appropriate current. Additionally, devices capable of higher energy, higher current milling cannot maintain a small beam diameter. These devices typically achieve a beam diameter as low as 1 mm, such as the Technoorg Linda Gentle Mill, manufactured by Technoorg Linda, Budapest, Hungary.
Another device used to prepare specimens is a focused ion beam, or FIB. FIB milling was originally developed for circuit editing in the semiconductor industry to cut and weld traces. In FIB milling, a small diameter, high energy ion beam is generated from a liquid metal source. Typically, the diameter of the ion beam is on the nanometer scale and the energy of the beam is on the order of 5-30 keV. In light of its small beam diameter, FIB milling may be used for very fine cutting applications. Additionally, because of this fine cutting capability, focused ion beam etching has also been used for other specimen preparation to create the electron transparent area. For example, FIB milling is often used to create TEM specimens from processed microelectronic wafers. One common example of such use of the FIB technique is known as an H-Bar sample. In an H-Bar sample, two trenches, approximately 20 micron wide, are cut into the top and bottom of a cleaved or ground section of a wafer, leaving an electron transparent area between the trenches. One problem with focused ion beam etching as used in TEM specimen preparation is, because of the high ion energy and/or mass, the FIB processes often damage the crystalline structure of the specimen, thereby causing amorphization. In addition, the metal ions tend to penetrate the specimen substrate, a condition known as implantation. Amorphization and implantation both adversely affect the quality of the TEM image that may be obtained from the specimen. Conventional ion milling may be used to remove or remedy some of this amorphization and implantation. However, because the ion beam used in conventional ion milling is typically on the order of 1 mm and the trenches in an H-Bar sample are on the order of 20 microns, the ion beam will often remove some specimen material from the edges surrounding a trench and deposit that material in the trench. This problem, known as redeposition, also adversely affects the quality of the TEM image obtained from the specimen.
A variety of other methodologies are utilized either with or without the use of a FIB. These include grinding and polishing a specimen into a relatively thin, wedge shaped orientation, which may then be viewed at the thin edge of the wedge or carved directly from the face of a substrate utilizing the FIB. In one particular methodology, a thin slice of material is removed from a solid substrate by removing a trench of material immediately adjacent the thin slice or section of the substrate material to be viewed. The thin slice is protected during the milling of the trench and is subsequently removed once the area around it has been cleared by cutting the thin, roughly rectangular section away from the surrounding substrate walls.
In any of the previous examples of specimen preparation, the use of mechanical grinding and cutting techniques, as well as cutting and thinning through the use of the FIB, results in relatively localized amorphous damage to the specimen as described above. A number of techniques have been utilized in the prior art to alleviate both the creation of the damage to the specimen during its initial preparation, as well as remove the damage created by that preparation. Such techniques include the use of gas plasma, as disclosed in Fischione, U.S. Pat. No. 5,633,502. Various alternative preparation techniques, as described above, have further been developed for the purpose of exposing an appropriate area of interest of the specimen in such a manner that the physical separation of the sample section containing the area of interest from the surrounding substrate layer and the thinning of the sample take place in an area spatially removed from the particular area of interest.
As will be apparent to those skilled in the art, the use of lower energy ions for less abrasive mechanical techniques would minimize specimen damage, however, the ability to solely utilize these techniques while retaining a reasonable preparation time and treating a given area of the specimen without redeposition has not been resolved.
The requirement of electron transparency therefore necessitates the utilization of some electrical, chemical, thermal or mechanical preparation methodology before the exposure of the surface at the precise area of interest. Prior ion milling devices have been utilized in a variety of ways to achieve these same purposes. Typical ion milling energies and prior art devices, however, range from 0.5 to 10 keV. Alternative methodologies for reducing the impact damage of such traditional ion sources include the use of milling at low angles in order to reduce the direct impact of the ions utilized for milling on the specimen surface and for the more careful and controlled removal of specimen material from that surface. Ion Mill Model No. 1010, currently manufactured by E. A. Fischione Instruments, Inc. of Export, Pa., is a typical example of the prior art mill. It incorporates the use of hollow anode discharge, or HAD, ion sources, which are mounted adjacent to a tilting and rotating specimen stage. The use of the tilting and rotating specimen stage allows for the manipulation of the specimen relative to the HAD ion sources and for projecting and moving the ion beam across the surface of the specimen. While ion mills of the prior design have been effective, new developments in nanotechnology, electron microscopy and the continued sub-miniaturization of the specimen areas of interest have necessitated further improvements in both the magnification power of the transmission electron microscopes as well as the need for reduction of specimen damage during preparation. At higher levels of magnification, the damage from prior art preparation techniques threatens not only to overwhelm the field of view in specimen imaging, but also to produce a variable and unpredictable modification of the specimen structure. What is lacking in the art, therefore, is a methodology of thinning a specimen to electron transparency which provides both time efficient gross specimen preparation and thinning capability, and finely controlled finishing capability, while minimizing damage to the specimen through the use of both high and low energy ion beams having a relatively small beam diameter.
What is further lacking in the art, moreover, is the ability to prepare the specimens with minimal damage utilizing a variety of techniques or devices under carefully controlled conditions of temperature and vacuum. A number of devices are currently identified in the prior art which provide many of the features identified above, but which are provided only in discreet implementations or devices without regard to the condition of the specimen being transferred between such preparation devices or intermediate such techniques.